1. Field of the Invention
The present invention relates to a sintering aid for alkaline earth metal titanates and in particular, to a sintering aid which avoids the use of toxic components, and which can be used with water-based binder systems based on polyvinyl alcohol.
2. The Prior Art
Barium titanate and its isostructural derivatives are ceramic materials which are widely used as dielectric materials for the manufacture of capacitors. Dense barium titanate cannot be formed by sintering at temperatures below approximately 1250.degree. C. When such materials are used for the manufacture of multilayer ceramic capacitors, this high sintering temperature and the necessary presence of oxygen entails the use of electrodes made from expensive precious metals such as palladium. A reduction in the manufacturing cost of such capacitors can be made by the substitution of silver for some of the palladium used in the electrodes. An alloy containing a ratio of 70:30 parts of Ag:Pd is commonly used, but this requires the sintering temperature, to be lowered to below 1150.degree. C. To achieve this reduction in processing temperature additions of materials referred to as "sinter aids" are made to the ceramic material.
Previously, barium titanate has been sintered at temperatures of 950.degree. to 1150.degree. C. using as sintering aids glasses containing one or more of the elements bismuth, lead, cadmium and boron. Although relatively dense products are obtained, the products suffer from the disadvantages that the fluxes used reduce the permittivity of the ceramic by dilution and/or by forming secondary phases which are detrimental to the electrical or mechanical properties of the sintered body. Furthermore, lead and cadmium are toxic, whilst boron causes gels to form with water-based binder systems containing poly(vinyl alcohol), which restricts its use to binders compatible with organic solvents.
It is also well known in the art that incorporating other aliovalent ions into the BaTiO.sub.3 perovskite lattice forming solid solution and/or composite materials can both shift, broaden and flatten the Curie peaks--forming commercially useful dielectrics. Numerous examples can be found in the literature of blends containing one or more of, for example, calcium and strontium titanates or zirconates and calcium, magnesium or bismuth stannates. In addition to "peak shifters," commercial formulations typically contain other additives, e.g., Nb.sub.2 O.sub.5, MnO.sub.2, CaO and NiO, to modify grain growth characteristics, loss factor or insulation resistance and such like.
U.S. Pat. No. 4,222,885 discloses a method for producing a dielectric having a perovskite structure consisting essentially of one or more of alkaline earth titanates, alkaline earth zirconates, alkaline earth stannates and mixed crystals thereof in which an admixture of eutectic-forming compounds, which cannot in practice be built into the perovskite lattice, are added to the perovskite and the mixture sintered at a partial oxygen pressure of 0.2 to 1 Bar at a temperature in the range of from 1000.degree. to 1250.degree. C. The eutectic forming compounds are compounds capable of forming CuO.MeO.sub.x when sintered in an oxygen atmosphere, MeO.sub.x being the oxide of at least one element of group III, V, VI or VII of the Periodic Table of the Elements
U.S. Pat. No. 4,244,830 discloses a method of forming a dielectric having a perovskite structure which is similar to that disclosed in U.S. Pat. No. 4,222,885. The method comprises adding to the perovskite eutectic-forming compounds which are capable of forming CuO.Cu.sub.2 O or CuO.Cu.sub.2 O.Me.sup.IV O.sub.2 when sintered in an oxygen atmosphere, where Me.sup.IV O.sub.2 is at least one oxide of an element of group IV of the Periodic Table of the Elements. Sintering is carried out at a partial oxygen pressure in the range of from 0.2 to 1 Bar at a temperature in the range of from 1000.degree. to 1250.degree. C.
U.S. Pat. No. 4,283,753 discloses a dielectric ceramic material comprising a major portion of barium titanate, the positive ions in the body consisting of:
a number N.sub.D of large divalent ions selected from Ba, Pb, Ca and combinations thereof, PA1 a number N.sub.Q of small quadravalent ions selected from Ti, Zr, Sn, Mn and combinations thereof, PA1 a number N.sub.X of donor ions capable of having a valence of greater than +4 as a small cation selected from Bi, Nb, Sb, W, Mo and combinations thereof, and PA1 a number N.sub.CC of charge compensating acceptor ions capable of having a valance of +1 as a large cation selected from Cd, Zn, Cu, Li, Na and combinations thereof, and PA1 a number of N.sub.G of glass-forming ions selected from B, Si, Ge, P, V and combinations thereof PA1 wherein the various numbers N.sub.D, N.sub.Q, N.sub.X, N.sub.CC and N.sub.Q are related by certain equations outlined in U.S. Pat. No. 4,283,753.